Generally, in the MRI, an echo signal at each grid point on the k space (space which is called a “measurement space”) is collected by Cartesian sampling in which sampling parallel to the frequency encoding direction is repeated in the phase encoding direction. In Cartesian sampling, an echo signal is sampled repeatedly while changing the amount of phase encoding.
However, so-called body motion artifacts are caused in the phase encoding direction due to random body motion or periodic motion, such as the pulse, of an object. This occurs because a random phase change is added to an echo signal and this echo signal is not disposed at the correct position at the time of Fourier transform in the phase encoding direction when imaging a target whose position changes during imaging.
In order to reduce body motion artifacts due to periodic motion, there is a synchronous imaging method executed by synchronizing a repetition time (TR) of the imaging procedure, which is called an imaging sequence, with a periodic biological signal acquired from an object. As the main synchronous imaging method, there are a respiratory synchronization method which suppresses body motion artifacts caused by breathing and an electrocardiographic synchronization method which suppresses body motion artifacts caused by the movement or beating of the heart.
In the synchronous imaging method, however, the degree of freedom in setting an imaging parameter is reduced since the repetition time (TR) is restricted to the physiological period of the body. For example, when acquiring a T1-weighted image, it is preferable to set TR to about 500 msec in the MRI apparatus of 1.5 T. In the electrocardiographic synchronization method, however, TR is set in the range of 900 msec to 1 sec since it is necessary to make the sequence synchronize with an interval of the cardiac cycle of the object. Therefore, if the electrocardiographic synchronization method is used together with acquiring the T1-weighted image, it is not possible to set an optimal imaging parameter. As a result, it is difficult to acquire the correct contrast.
In order to reduce body motion artifacts regardless of whether the body motion artifacts are caused periodically or randomly, a non-Cartesian sampling method has been proposed. As examples of the non-Cartesian sampling method, a radial method (for example, refer to NPL 1), a hybrid radial method (for example, refer to NPL 2), and a spiral method (for example, refer to NPL 3) are known.
The radial method is a technique of acquiring echo signals required for reconstructing one image by performing radial sampling while changing the rotation angle with approximately one point (generally, the origin) of the measurement space as the rotation center. Since imaging is completed every rotation angle, it is difficult to cause artifacts. Moreover, since sampling is radially performed, the central portion of the measurement space is repeatedly measured. Accordingly, artifacts are less noticeable due to the addition effect. Furthermore, even when artifacts are caused, the artifacts are scattered within the image since sampling is not performed in a specific direction. Accordingly, artifacts are less noticeable compared with the Cartesian sampling method.
In addition, the hybrid radial method is realized by combining the radial method with phase encoding. In the hybrid radial method, sampling is performed by dividing the measurement space into a plurality of blades with different sampling directions and the phase encoding is performed within the blades. The hybrid radial method has not only the characteristic of the radial method but also a characteristic that it can be easily applied to the sequence of a multi-echo method which acquires a plurality of echo signals by one application of the high frequency magnetic field. In addition, an FSE method, an echo planar method, and the like are known as examples of the multi-echo method applied to the hybrid radial method.
The spiral method is a technique of acquiring echo signals required for reconstructing one image by performing spiral sampling while changing the rotation angle and the radius of rotation with approximately one point (generally, the origin) of the measurement space as the rotation center. The spiral method is applied as a high-speed imaging method since less time is wasted when filling the measurement space and the data can be efficiently collected. In addition, the spiral method is characterized in that a gradient magnetic field pulse waveform used when reading an echo signal is not a trapezoidal wave but a combination of a sine wave and a cosine wave and accordingly, the gradient magnetic field pulse waveform is efficient for the gradient magnetic field system and there is less noise when applying a gradient magnetic field.